This invention relates generally to glass-ceramic cooktop appliances and more particularly to methods and devices for sensing properties relating to the appliance, such as temperature of the glass-ceramic cooktop surface, properties of cooking utensils placed on the cooktop, and properties of the utensil contents.
The use of glass-ceramic plates as cooktops in cooking appliances is well known. Such glass-ceramic cooktops have a smooth surface that presents a pleasing appearance and is easily cleaned in that the smooth, continuous surface prevents spillovers from falling onto the energy source underneath the cooktop.
In one known type of glass-ceramic cooktop appliance, the glass-ceramic plate is heated by radiation from an energy source, such as an electric coil or a gas burner, disposed beneath the plate. The glass-ceramic plate is sufficiently heated by the energy source to heat utensils upon it primarily by conduction from the heated glass-ceramic plate to the utensil. Another type of glass-ceramic cooktop appliance uses an energy source that radiates substantially in the infrared region in combination with a glass-ceramic plate that is substantially transparent to such radiation. In these appliances, a utensil placed on the cooktop is heated partially by radiation transmitted directly from the energy source to the utensil, rather than by conduction from the glass-ceramic plate. Such radiant glass-ceramic cooktops are more thermally efficient than other glass-ceramic cooktops and have the further advantage of responding more quickly to changes in the power level applied to the energy source. Yet another type of glass-ceramic cooktop appliance inductively heats utensils placed on the cooktop. In this case, the energy source is an RF generator that emits RF energy when activated. The utensil, which contains an appropriate material, absorbs the RF energy and is thus heated.
In each type of glass-ceramic cooktop appliances, provision must be made to avoid overheating the cooktop. For most glass-ceramic materials, the operating temperature should not exceed 600-700.degree. C. for any prolonged period. Under normal operating conditions, the temperature of the glass-ceramic plate will generally remain below this limit. However, conditions can occur which can cause this temperature limit to be exceeded. Commonly occurring examples include operating the appliance with no load, i.e., no utensil, on the cooktop surface, using badly warped utensils that make uneven contact with the cooktop surface, and operating the appliance with a shiny and/or empty utensil.
To protect the glass-ceramic from extreme temperatures, glass-ceramic cooktop appliances ordinarily have some sort of temperature sensing device that removes power from the energy source if high temperatures are detected. In addition to providing thermal protection, such temperature sensors can be used to provide temperature-based control of the cooking surface and to provide a hot surface indication, such as a warning light, after a burner has been turned off.
One common approach to sensing temperature in glass-ceramic cooktop appliances is to place a temperature sensor directly on the underside of the glass-ceramic plate. With this approach, however, the temperature sensor is subject to the high burner temperatures and thus more susceptible to failure. Moreover, direct contact sensors are limited in the area of the glass-ceramic plate that they can monitor and can fail to detect hot spots that may form on the glass-ceramic plate. The heating portion of the glass-ceramic plate (i.e., the portion directly above a burner) tends to get hottest at points not covered by a utensil. Hot spot formation can occur anywhere on the heating portion of the glass-ceramic plate due to poor utensil placement with respect to the burner or by using undersized or warped utensils. And since direct contact sensors do not monitor the entire heating portion of the glass-ceramic plate, the affect of hot spots on the integrated temperature of the entire heating portion may not be accurately measured.
These problems also exist for sensors other than glass-ceramic temperature sensors. This would particularly include sensors that are designed to "look" through the glass-ceramic plate to detect characteristics of a utensil placed on the cooktop, such as the temperature, size or type of the utensil, the presence or absence of the utensil, or the properties, such as boiling state, of the utensil contents.
Accordingly, there is a need for a sensor assembly for glass-ceramic cooktop appliances that can sense cooktop related properties without being subjected to the hot burner environment and that can monitor a relatively large area of the glass-ceramic plate.